1. Field of the Invention
This invention relates to a method of making a magneto-optical recording medium such as a magneto-optical disk used for large-capacity data storage or the like. This invention particularly relates to a method of forming thin films such as a recording medium layer and a protective layer by sputtering on a substrate of the magneto-optical recording medium.
2. Description of the Prior Art
Optical recording media have various advantages such that they can record information at a high density, have a large capacity, and need not be contacted with a head. Among such optical media, the magneto-optical recording medium has attracted particular attention for enabling easy erasing and rerecording.
The magneto-optical recording medium is composed of a magnetic material as a recording medium, and records information based on changes in magnetization of the magnetic material. For example, an amorphous magnetic material composed of a combination of a rare earth metal (RE) such as Gd, Tb or Dy with a transition metal such as Fe, Co or Ni is used as the magnetic material. The magnetic material is used in a layer form as a recording layer. The magnetic material, particularly the rare earth metal contained therein, is readily oxidized. When the magnetic material is oxidized, magnetic characteristics such as coercive force thereof deteriorate. Therefore, in order to isolate the recording layer from ambient air, a protective layer formed of a dielectric material or the like is formed on the recording layer by use of a sputtering method or the like.
However, the sputtering process has the drawback that free oxygen given rise to in the course of sputtering for the formation of the protective layer clings to the surface of the recording layer and penetrates through the recording layer, thereby oxidizing the magnetic material. As a result, oxidization is promoted in the course of formation of the protective layer for the purpose of preventing oxidization.